An imaging device, or sensor, is a photosensitive electronic component used to convert an electromagnetic ray into an analog electrical signal. This signal is then amplified and digitized by an analog-digital converter and finally processed to obtain a digital image. The imaging device makes use of the photoelectric effect. An imaging device usually comprises photosites arranged in a matrix, each photosite corresponding to a pixel of an image.
A photosite comprises at least one photosensitive zone, notably a photodiode, and a zone for reading the charges accumulated in the photodiode. The photons sensed by a photodiode of a photosite are converted into electron/hole pairs. The charges of a first type, for example, the holes, created in the photosensitive zones are drained at depth to the substrate (p+), the charges of a second type, for example, the electrons, are stored in the photosite before being read by virtue of an electronic system.
Usually, this electronic system, which controls the photodiode, comprises, notably when the photodiode is a fully depleted photodiode, a transfer transistor controlling the transfer of the charges stored in the photodiode to a zone for reading charges. This zone for reading charges forms a sensing node to which is connected typical electronic read element comprising notably a read transistor.
A photodiode operates in a cycle comprising at least one integration step, a measurement step, and a reset step. The integration step corresponds to the photogeneration of charges and their accumulation during the exposure of the photodiode to light. The measurement step corresponds to the generation of a signal depending on the quantity of photogenerated charges accumulated in the photodiode. The reset step corresponds to the elimination of the photogenerated charges.
The light intensity received by a photosite corresponds to the number of photons received. The light intensity is directly proportional to the number of photons that can react with the photosensitive zone, namely the photodiode. The weaker light intensity reduces the number of incident photons.
In order to obtain a significant signal, i.e. a signal that stands out from the electronic noise, it is necessary to receive a sufficient number of photons. For a measurement to be carried out from a light ray of low intensity, the exposure time is longer than to obtain an equivalent measurement from a light ray of higher intensity.
One approach for taking a measurement under low light intensity with an imaging device with a matrix network of photosites may include taking the measurement after a relatively long integration time. However, that may require knowing the light intensity of the incident ray in advance. Specifically, when the incident ray has a relatively high intensity, the photosites of the imaging device may be saturated with charges after the integration time.
Another approach includes using an imaging device of which several photosites are coupled together in order to increase the area sensitive to the incident ray of low intensity. However, such a device may increase the size of the pixels and reduce the definition.